1. Field of the Invention
This invention relates to a method of manufacturing a radiation detector having a function of detecting radiation including X-rays, gamma-rays, light, or the like for use in the medical, industrial, and nuclear fields. More particularly, this invention is directed to a technique in which a radiation-sensitive detecting layer is formed of a semiconductor which is formed of polycrystals.
2. Description of the Related Art
As conventional materials of a high-sensitive radiation detector, various semiconductor materials, especially a crystal of CdTe (cadmium telluride), ZnTe (zinc telluride), or CdZnTe (cadmium zinc telluride) have been studied and developed, and have been partially commercialized. However, in order to apply this to a radiation detector or a radiographic apparatus for medical diagnosis, it is necessary to form a radiation conversion layer having a large area (e.g., of a 20 cm square or more). It is not practical in technique and in cost to form a crystal having such a large area. Thus, a method has been disclosed of forming a polycrystalline film or a polycrystalline lamination film by close spaced sublimation. See, for example, Japanese Patent Publication No. 2001-242255.
In a smaller radiation detector using a CdTe bulk monocrystal, it has been known as effective to dope it with zinc (Zn) for decreasing a leakage current and to dope it with halogen, such as chlorine (Cl), for enhancing carrier mobility and thus enhancing detection characteristics. As a Cl-dope technique for a CdTe or CdZnTe polycrystalline film by close spaced sublimation, a method of forming a polycrystalline film or a polycrystalline lamination film has been disclosed by vapor deposition or sublimation using a mixture of a first material containing at least one of CdTe, ZnTe and CdZnTe and a second material containing at least one of CdCl2 (cadmium chloride) and ZnCl2 (zinc chloride) as a source. See, for example, Japanese Patent No. 4269653.
In the Cl-dope technique in the above-mentioned Japanese Patent No. 4269653, however, Cl compounds, such as CdCl2 and ZnCl2, are to be previously consumed at a lower melting point and a higher steam pressure rather than that of CdTe, ZnTe, or CdZnTe. Accordingly, chlorine (Cl) is no longer supplied during film formation, and only a portion adjacent to a substrate interface is doped with Cl in early film formation. Such problem has appeared.
As shown in FIG. 8, crystal particles are smaller adjacent to the substrate interface doped with Cl, whereas they are larger in a portion not doped with Cl. Thus, it has been confirmed that crystal particles cannot be uniform. The non-uniform crystal particles cause non-uniform characteristics (e.g., a leakage current or sensitivity) in each pixel. In addition, temporal fluctuations become wider. These serving as a noise source leads to difficulty in sensitivity correction and deterioration in image characteristics (detection efficiency).
Accordingly, as in the third aspect of the above-mentioned Patent Publication No. 4269653, it has been suggested that the polycrystalline film or the polycrystalline lamination film is formed by the vapor deposition or sublimating with the above-mentioned source, and then is additionally doped with Cl. In this case, additional dope with Cl can achieve protection of the crystal particles. On the other hand, a diameter itself of the particles largely grown-up cannot be decreased. As a result, uniform crystal particles cannot still be obtained.
Moreover, regarding that Cl is not supplied during the film formation, it is also conceivable to increase a ratio of the second material represented by CdCl2 (cadmium chloride) and ZnCl2 (zinc chloride) so as to supply Cl continuously until the end of the film formation. On the other hand, it has been obvious from experimental data in FIG. 9 that the second material is gone firstly during the film formation even if the ration of the second material increases.
FIG. 9 illustrates experimental data showing of variations with time in pressure within a chamber for every component using CdCl2 only as a source, whereas FIG. 10 illustrates experimental data showing of variations with time in pressure within a chamber for every component using CdCl2 as a source and HCl as an additional source.
It can be seen from the experimental data that among the components within the chamber, components (Cl, H2) of HCl (hydrogen chloride) decreases with time when a polycrystalline film or a polycrystalline lamination film is formed by close spaced sublimation method only with the source as shown in FIG. 9. On the other hand, it can also be seen that a component HCl among the components within the chamber does not decrease with time when HCl for another Cl component other than the source is supplied as an additional source.
This invention has been made regarding the state of the art noted above, and its object is to provide a method of manufacturing a radiation detector that can achieve uniform crystal particles and uniform detection characteristics.